While it might seem crazy to add a variable HT/B+ to a One Watt amp, that is exactly what I have done and I am very pleased with the results. You may recall from an earlier post that I found that even a “puny” 1W amp was too loud in some circumstances; this post describes the solution that I chose.
Denis Cornell takes his Romany Plus amp down to 50mW and, while I was skeptical that 50mW would be a useful amount of power, it had to be worth a try. In fact I decided to go for 100mW as it should sound half as loud as 1W. The white, chicken-head knob that you can see in the picture allows the power output to be scaled from 10% to 100%. The black dots around the knob indicate the positions of eighth-, quarter-, half- and full-power. Click here to view the schematic (the new section is around the Power MOSFET Q1).
I only completed the work on it today but I am really impressed. At the lower power settings there is no clean headroom but that’s the point. You reduce the HT/B+ to get distortion at ultra-low volumes; If you want clean you can increase the HT/B+ and reduce the gain. The tone of the amp doesn’t really seem to change as you change the power output although down at 100mW it loses a little sparkle; I prefer 0.25W.
I won’t go into the details now but I will be updating the Little One website soon and you can read the final section of Merlin Blencowe’s book on Power Supplies if you need any information about the MOSFET voltage follower.
Little One was only ever intended as a prototype but I’ve been using it for years. It looks tatty and the circuit has grown organically to the point it will be tricky to change as you can see from the picture. It’s about time I rebuilt it.
Update: Having played the modified amp for a couple of weeks now, I think Denis Cornell got it 100% right (hardly a shock). If you’ve got an efficient speaker, you should be able to run your amp at 50mW. This will allow you to get some bite and crunch at the same volume as an acoustic guitar. To do this change R24 to 130K (120K will do).